X-ray Crystallographic Visualization of a Nucleation and Anisotropic Growth in Thiolate-Protected Gold Clusters: Toward Targeted Synthesis of Gold Quantum Needles.

Understanding and controlling the nucleation and growth processes of gold clusters are crucial for advancing the nucleation theory and targeted cluster synthesis. While mass spectrometry has revealed the intermediate species formed during the growth process, the overall structural evolution remains unclear due to a lack of crystallographic information. In this study, we examined a new synthetic method for thiolate-protected gold clusters in their embryonic stage. In this method, only partial Au(I) precursors are reduced by a minimal amount of mild reductant in the presence of a substoichiometric amount of thiol. Under optimized conditions, we obtained a series of small gold clusters, including Au(SCTMS), Au(SCTMS), Au(SCTMS), Au(SCTMS), Au(SCTMS), Au(SCTMS), and Au(SCTMS) (TMSCS: trimethylsilylmethanethiolate), and successfully determined their geometric structures via single-crystal X-ray diffraction analysis. Unexpectedly, the synthetic yield of Au(SCTMS) with an icosahedral Au core was very low despite the stability of this composition. The obtained structural information suggests that under our synthetic conditions the dominant process is the assembly of triangular Au and tetrahedral Au units, each with two electrons, in an anisotropic structure followed by passivation with unreduced Au(I)-SCTMS complexes. Notably, Au(SCTMS) and Au(SCTMS) have a pencil-shaped Au core with an Au width and exhibit strong absorption and emission in the near-infrared region. Due to their extremely thin diameter and quantized electronic structures, we propose naming these anisotropic species "gold quantum needles". This study deepens our understanding of the cluster formation mechanism at the atomic level and provides a novel synthetic route for highly anisotropic gold clusters.